Prolactin (PRL) suppresses gonadal function in many species and is an important clinical cause of reproductive dysfunction. Although PRL acts primarily by inhibiting GnRH and gonadotropin release, the mechanisms by which this occurs are still unclear. The current proposal focuses on endogenous opioid and dopaminergic mechanisms leading to GnRH suppression in the hypothalamus. Specifically the regulation by PRL of beta-endorphin (beta-EP) and its precursor proopiomelanocortin (POMC) will be studied in the hypothalamus, and dopamine will be examined as a mediator of the effects of PRL on POMC. Mechanisms by which POMC is regulated by PRL and dopamine will be established by a coordinated study of POMC gene expression, precursor processing and peptide release and related to changes in GnRH and LH secretion. The effects of increased PRL (induced by peripheral or central intracerebroventricular infusion of highly purified PRL preparations) on POMC mRNA levels in the hypothalamus will be determined by a sensitive solution hybridization assay. Dopamine regulation of POMC gene expression will also be studied and dopamine antagonism will be utilized to determine if the effects of PRL on POMC are mediated by dopamine. The effects of PRL on the posttranslational processing of POMC will be studied by HPLC and well characterized RIAs for the major POMC peptide products in brain. It is hypothesized that the processing of POMC may be an important regulator of beta-EP bioactivity which can be influenced by the simultaneous release of other peptides from the same precursor or by C-terminal processing of beta-EP itself. PRL regulation of POMC peptide and GnRH release will be studied in the perfused hypothalamus in vitro. Antagonism of beta-EP and dopamine will be utilized to determine their respective roles in mediating PRL-induced GnRH and LH suppression in the rat. These studies will also be extended to hyperprolactinemic patients in a test of the ability of naltrexone, a long-acting oral opioid antagonist to block the suppressive effects of PRL on pituitary-gonadal function. The effects of chronic naltrexone administration will be studied as a potential therapeutic modality to restore reproductive function in these patients.